This March 13, 2014 file photo shows cracks in the dry bed of the Stevens Creek Reservoir in Cupertino, Calif. The Obama administration is more certain than ever that global warming is changing Americans' daily lives and will worsen — conclusions that scientists will detail in a massive federal report to be released Tuesday, May 6, 2014. (AP Photo/Marcio Jose Sanchez, File)

A crew in Groveland (Tuolumne County) works on a backfire to fight August's Rim Fire, which at 400 square miles was the third-largest wildfire in state history. Wildfires are projected to increase sharply if emissions are not curtailed.

Washington -- Dwindling water for farms, longer fire seasons and coastal flooding of homes and businesses await California as climate change intensifies, according to a federal report released Tuesday that details how global warming is damaging every region of the country.

The third National Climate Assessment, compiled over four years by more than 300 scientists at the direction of Congress, said California's farm industry, which provides more than half the nation's fruits, nuts and vegetables, is particularly vulnerable. So are many cities along the coast, including San Francisco, that are already experiencing flooding at high tides as sea levels rise.

Competition for scarce water as the Sierra snowpack diminishes is expected to intensify among cities, farmers and the environment, the report says.

California had its warmest January on record this year. Temperatures nationally are expected to rise 2 to 4 degrees Fahrenheit over the next few decades, on top of a 1.5-degree warming since the late 1800s, most of it since 1970, the report says.

Obama's plans

The report sets up President Obama's politically perilous end run around Congress, starting with regulations he is expected to put in place next month that would clamp down on greenhouse gas emissions from coal-fired power plants.

Goaded by White House science adviser and former UC Berkeley Professor John Holdren and White House special adviser John Podesta, Obama plans to make addressing climate change a major push of his remaining time in office, even as control of the Senate rests with Democrats clinging to seats in the heart of the fossil fuel industry.

"The real bottom line of this report is that climate change is not a distant threat, but is already affecting every region of the country and key sectors of the economy," Holdren said. "This is the loudest and clearest alarm bell to date signaling the need to take urgent action."

Affecting lives

Holdren called the report "unprecedented in its comprehensiveness and detail." By looking at specific regions, in contrast to international reports that lump together climate change's effects by continent, the report is designed to show Americans how global warming is already affecting their lives.

Obama dropped the global-warming issue in his first term despite having Democratic control of Congress, pursuing health care reform instead. Then-House Speaker Nancy Pelosi of San Francisco narrowly pushed through cap-and-trade legislation to hold down carbon emissions, but Sen. Barbara Boxer, D-Calif., failed in her effort to get a similar bill through the Senate.

Now Obama is returning to the subject, forging ahead with plans to regulate greenhouse-gas emissions from coal-fired power plants over Republican opposition. Boxer obtained a green light from Podesta this year for her campaign to build public support for steps to combat global warming.

Big opportunity

Power plants produce a third of U.S. carbon dioxide emissions and represent the biggest opportunity to curtail warming, said Kevin Kennedy, director of the U.S. climate initiative at World Resources Institute, an environmental think tank. "Further delay will only accelerate climate change and raise the costs of addressing its impacts," Kennedy said.

House Republicans promised to push back. Rep. Fred Upton, R-Mich., chairman of the Energy and Commerce Committee, accused Obama of "advancing an agenda against affordable and reliable energy" that he plans to "unilaterally impose on the American people" at a high cost to jobs.

Podesta acknowledged that climate change is low on the list of voters' concerns, saying most people "don't feel that sense of urgency. This report can influence that."

Bad news for farms

The National Climate Assessment said a warmer climate will have an especially harsh effect on the produce grown in California, almost all of which has a high water content - such as grapes and tomatoes - and relies on irrigation.

"Winter chill periods are projected to fall below the duration necessary for many California trees to bear nuts and fruits," the report said, while warm-season vegetable crops in some areas "may not be viable under hotter climate conditions." At the same time, higher temperatures will increase crops' water needs.

The Southwest as a whole, where the population is expected to increase 68 percent by midcentury to 94 million people, is already short of water, the report said. As the region continues to dry, the assessment said, there will be "increasing competition among farmers, energy producers, urban dwellers, and plant and animal life for the region's most precious resource."

Wildfires are projected to increase sharply as soils dry, potentially doubling burned acreage in Northern California by the end of the century if emissions are not curtailed.

Rising seas

The report said sea levels are projected to rise from 1 to 4 feet by the end of the century, depending on how much ice melts in Greenland and Antarctica, threatening coastal power plants, sewage treatment plants, ports, airports, highways and other public infrastructure, as well as homes and businesses. More than half the nation's population lives in coastal counties.

The San Francisco and Oakland airports, and the Port of Oakland, are at sea level. Much of the East Coast is subsiding, putting coastal cities such as Miami, Boston and New York at high risk from extreme high tides and storm surges, the report said.

Holdren said the nation should cut future emissions by reducing the burning of fossil fuels and make investments to adapt to climate changes that are already baked in.

"This is not a problem about which we just have to wring our hands," Holdren said. "There are cost-effective measures we can take on both sides. We need to get on with it."

Warming West

Findings of the National Climate Assessment, released Tuesday:

Heat: Temperatures have risen since the 1950s in California, Nevada and the Four Corners states faster than at any other time in at least the past 600 years.

Wildfires: From 1970 to 2003, warming temperatures and drier conditions contributed to a 650 percent increase in forest acreage burned in wildfires.

Fire forecast: If greenhouse-gas emissions continue at current levels, annual acreage burned in wildfires in Northern California could double by the end of the century.

Snow: California and other Western states will see a drop in the water content of snow.

Agriculture: Drier weather will lead to less water for agriculture, which accounts for about 80 percent of water usage in the Southwest.

Sea level: A 16-inch rise in sea level on the California coast is expected, threatening the San Francisco and Oakland airports.